1. Field of the Invention
This invention relates to polyvinyl alcohol copolymers for use in paper coatings.
2. Description of the prior art
In the process of producing coated paper, paper coatings are applied to the surface of a paper product to obtain the coated paper. The paper coating can include various components, including polyvinyl alcohol. Other components may include calcium carbonate, clays such as hydrous aluminum silicates of the kaolin variety or hydrated silica clays, titanium dioxide, blanc fixe, lithopone, and zinc sulfide. Other components of a conventional paper coating composition can include, for example, mineral pigments, pigment binders, thickeners, humectants, and lubricants—all in various proportions. Paper coatings used to produce coated papers can include high percentages of solids, and such solids levels can approach 70%. Polyvinyl alcohol can be used as a clear coat and also as a co-binder with latex in coated papers. Typically in paper coatings applications polyvinyl alcohol is used and applied as an aqueous composition that typically comprises other components. It can be, and often is, desirable to use high solids paper coatings for more efficient production of coated paper.
However, polyvinyl alcohol (PVA) is difficult to prepare as an aqueous solution of more than 30% solids, and above this level dispersing the polyvinyl alcohol in water becomes very difficult and the resulting solution viscosity very high. To obtain even this degree of solution, an aqueous mixture that includes PVA is typically heated, or “cooked” to achieve complete solution and the resulting PVA solution is then added to a paper coating composition. In paper coatings applications, the amount of water required to deliver an aqueous PVA solution is detrimental, or at least undesirable, in the paper coating composition. It is much preferred for paper coatings applications to use as little water as possible and as a result mixtures having 70% or more of water are not desirable in paper coatings applications. The difficulties in dissolving polyvinyl alcohol in water can cause problems with the process of applying the coating. For example, there can be difficulties getting PVA dispersed into the coating mixture and evenly distributed on the paper surface with insoluble PVA.
U.S. Pat. No. 5,527,852 describes a particulate PVA slurry as a paper coating agent which can be prepared using PVA having an average particle diameter of from 0.01 to 1 μm. The PVA slurry described therein is said to be useful when applied to paper as a slurry wherein the PVA is substantially insoluble in cold water, and thereafter dissolves during the drying process to form a film on the surface of the paper.
U.S. Pat. No. 5,057,570 describes a PVA that is partially hydrolyzed, which can be added without additional water to a paper coating composition, and thereby avoids increasing the amount of water in the paper coating composition. However, it can be desirable to use fully hydrolyzed PVA rather than partially hydrolyzed PVA. Also, it can be desirable to use a higher molecular weight PVA polymer than described in U.S. Pat. No. 5,057,570. Each of these differences can reduce the solubility of the PVA in water, and thus negate the ability to dissolve the dry PVA in a high solids paper coating composition. It is also recognized that if the partially hydrolyzed PVA is not given time to fully dissolve, quality problems with the coating or the coated paper can be encountered.
US 2004/0186034 describes a water-soluble PVA copolymer film composition.
It can be desirable to dissolve a fully hydrolyzed PVA having a relatively high degree of polymerization in a high solids paper coating composition.